Single photon emission (SPE) imaging is a known medical imaging technique. It involves injecting radiopharmaceutical substance into a patient's body and evaluating the distribution of the radiopharmaceutical substance, which is indicated by the distribution of gamma rays emitted from within the patient's body.
A radiation-detecting system, often referred to as Gamma camera, detects those gamma rays. Gamma camera detects gamma rays emitted from the radiopharmaceuticals substances, and the data acquired is analyzed to form an image representing the distribution of concentrations of the radiopharmaceutical substance within a specific body area.
Several modalities of SPE imaging are in use. One of them is Single Photon Emission Computerized Tomography (SPECT). In this technique the gamma camera rotates around the region of interest of the patient's body, and data are collected at several angular positions (angular projections). A fully three dimensional image is formed. SPECT is considered to be a very useful technique and a good tool for obtaining diagnostic information, however it requires the collection of large number of emitted photon (large statistics) and this means that in order to obtain the required number of photons, a long acquisition time is necessary. Long acquisition time means that the patient is subjected to a relatively long period of discomfort and furthermore, making the overall number of patients who can be imaged in a given time relatively small—a feature that many medical institutes and hospitals regard as extremely unfavorable and undesirable.
Gamma camera generally comprises a photon detector crystal coupled with a plurality of photomultiplier tubes, or an array of solid-state detectors combined with position logic circuits and data analysis apparatus. A collimator for limiting the angle of incident gamma rays is incorporated with the gamma camera. Collimators are used to limit the detection of photons to a predetermined range of incidence angles (photons with greater incidence angles are absorbed by the collimator septa). A collimator typically includes thousands of square, round or hexagonal parallel channels, through which, and only through which, gamma rays are allowed to travel and reach the detector. Generally, a parallel-hole collimator is in use, however various other arrangements may be used.
As gamma rays are emitted from the radiopharmaceutical substance, they travel through the collimator, unabsorbed and interact with the detector, which is placed directly adjacent to the collimator. The interactions of the gamma rays with the detector crystal create flashes of light in a process called scintillation. The scintillation light is preferably detected by an array of photomultiplier tubes, which are normally coupled to the back of the crystal. Photomultiplier tubes are used when a very small amount of light is emitted in scintillation. The output signals from the photomultiplier tubes are electric pulses, proportional to the energy of the gamma rays. The electric pulse output is received by position logic circuits, which determine the position where the scintillation event had occurred on the detector. Similarly, in solid-state detectors including solid-state crystals, the incident photons produce electric current corresponding to the energy of the incident photon in the specific location of incidence. This current is picked up by electrodes coupled to the solid-state crystals and is processed. The data is processed by position logic circuits and is transferred to a processing computer in order to process the data into readable image of the spatial distribution of the radiopharmaceutical substances within the patient's body.
The main limitations to the quality of SPECT images come from limited number of registered gamma ray photons and the geometric resolution of the collimator. Intrinsic resolution of the detector plays a small role.
One of the most important components of current SPECT and Planar gamma camera is the collimator, which in practice defines system resolution and system sensitivity. The narrower the angular acceptance-range of the collimator, the better the resolution, but also the fewer the number of collected photons. Resolution is thus limited by sensitivity, and vice versa. High-resolution collimators typically used in the art reject photons arriving to the detector at angles larger than about 2 degrees (relative to the direction perpendicular to the detector from surface), while high-sensitivity collimators reject those at angles larger than about 3 degrees. The camera spatial resolution depends on the geometric resolution of the collimator, and degrades with distance between the surface of the collimator and the organ being imaged. The system's resolution, which depends on the distance is called Line (or point) Spread Function (LSF) of the collimator.
The system's sensitivity has a very weak dependency on the distance between the collimator and the emitting object.
In cameras used in the art, in order to get the best resolution for a given application (and with a given collimator) the distance between the collimator and the human body, which emits the photons, was kept as small as possible. Therefore, the position of the detector relative to the patient table is adjusted for each patient according to its physical dimensions.
Thus, very costly mechanical and electronic features are incorporated into a typical SPECT/Planar camera used in the art in order to enable the detector/collimator ensemble to get as close as possible to the patient's body. The detectors on these cameras are supported by a mechanical articulation device having multiple axis of motion. In most SPECT cameras, this usually includes the possibility to rotate the detector and to move it towards the patient (In) and away from the patient (Out). In few SPECT cameras with two detectors, where the in-out motion is disabled or restricted, the patient is brought to close proximity to the detector using elaborate table motion which includes up-down as well as left-right motion.
Some of the clinical procedures are done with a fixed radius of rotation, namely: the radius of the detectors does not change during the SPECT scan. The radius of rotation in this mode is determined for each patient and it mostly depends on the physical dimensions of the patient. In other modes of operation that exist in modern design, the radius of rotation for all or each of the detectors is changed as a function of the angular position.
These features are sometimes termed “body contouring”. During the scan, the detectors, while orbiting the patient's body get close to the body and follow the contour of the body while acquiring the data. In some of the SPECT gamma camera the “body contour” is designed as a pre-study (or learn mode) feature, namely; the radius for each rotating angle is studied before the scan followed by the real study with a radius dependent orbit. In other SPECT gamma cameras this is an on-line automatic feature that senses the patient contour and determines the closest possible radius for each angle.
The mechanical motion unit of the detector enables an “In and out” motion of the detector and collimator, i.e., it enables the detectors to get close to the body and to retract in areas where the patient body circumference is larger.
In a dual head camera, this in-out motion may be independent for each detector or the motion can be synchronized between the two heads and the patient bed.
Some dual head cameras are built wherein their two detectors are arranged so that their surfaces are substantially at right angle to each other during the scan. This configuration, known as “L mode” is the preferred configuration for cardiac examination. In this configuration, the independent motion of the detector is inhibited by the necessity to avoid collision between the detectors. Often the two detectors are fixed to each other and are moved as one unit. Performing body countering in L mode cameras is more complex and usually requires side-to-side motion of the bed.
In many modern systems one may find an automatic electronic element that keeps and controls the distance between the detector and the patient's body to a minimum while scanning the patient contour in either SPECT or Planar application.
The mechanical parts are costly and have many safety features built into them in order to avoid collision with the patient body while getting close to the body, i.e., while performing the in-out motion in orbit. Furthermore, these features increase the complexity of the electromechanical system and the complexity of the software that controls the motion, thus reducing the reliability of the camera.
The need for in-out motion of the detector and collimator ensemble does not permit many system design configurations that may have been more efficient than the classical single/dual/triple head configurations.
Some gamma camera systems of four detector-heads are known in the art. These systems are with a relatively very small Field of View (FOV) and are dedicated for brain applications. The typical sizes of the detectors are 25×25 cm2.
The process of recovering the three dimensional image from the acquired data is called SPECT reconstruction. Various types of SPECT reconstruction algorithms exist in the art. Generally these algorithms belong to one of two types:
1) Non-Iterative Reconstruction Methods:
Non-iterative (direct) reconstruction algorithms such as Filtered Back Projection (FBP) algorithms typically approximate the path of the gamma rays impinging the detector, by assuming parallel rays, thereby ignoring the LSF effect in the reconstruction and provide images in which the resolution is dominated by the collimator LSF effect.
2) Iterative Reconstruction Methods:
Iterative reconstruction algorithms search for solution that matches the acquired data. One of these algorithms is known as the Maximum Likelihood Expectation—Maximization (MLEM) method which attempts of find the solution that most matches the acquired data, based on the likelihood principle. Another method, which has an underlying similar mathematics is the Ordered Subset Expectation Maximization reconstruction (OSEM). This method was developed in order to reduce the computation time of the MLEM, which is computation intensive. Other iterative reconstruction algorithms include the block iterative reconstruction methods.
There are two main approaches to iterative reconstruction: Noise suppression and Resolution Recovery/Wide Beam Reconstruction. The most commonly used iterative reconstruction algorithms, such as OSEM yields noise suppression. These methods typically assume parallel rays, thereby ignore the LSF effect.
The Resolution recovery (RR) and/or Wide Beam Reconstruction (WBR) iterative methods (RR/WBR for short) allow for a priory knowledge of different physical dimensions of elements that exist in the data acquisition system to be integrated into the solution-measurements matching process. Accounting for the LSF effect together with the consideration of physical dimensions of the elements that control the effect, allow for the implementation of a mathematical solution to reduce or eliminate this effect.
The RR/WBR algorithms for compensation for the LSF effect in SPECT and/or Planar gamma camera are known in the art.
US Patent published patent application US-2003-0208117 (published on 6 Nov. 2003), titled “SPECT GAMMA CAMERA” which is incorporated herein by reference, discloses the WBR iterative reconstruction method and includes references for other iterative methods known in the art.
Currently both approaches to iterative reconstruction are focused on enhancing image quality by improving image resolution and reducing the noise. None of these approaches challenged the gamma camera mechanical design.
U.S. Pat. No. 5,554,848 (Hermony, et al. Sep. 10, 1996) titled “Gantry for nuclear medicine imaging systems” demonstrated the complexity of the gantry of nuclear cameras used in the art.
U.S. Pat. No. 5,486,700 (Silberklang, et al. Jan. 23, 1996) titled “Proximity controls for gamma camera” discloses a proximity controls for controlling the proximity of a gamma camera to a patient during a scan of the patient.
U.S. Pat. No. 5,777,332 (Lonn, et al. Jul. 7, 1998) titled “Automatic patient alignment during nuclear imaging body contour tomography scans” discloses a methods and systems for performing a tomographic scan which allow an operator to define a non-circular orbit so that a detector can be positioned close to a patient at each view.
U.S. Pat. No. 5,444,252 (Hug, et al. Aug. 22, 1995) titled “Adjustable dual-detector image data acquisition system” discloses an improved image acquisition system which allows the angular displacement between two detectors and a patient table capable to be displaced vertically and horizontally from a lateral axis to allow the body of a patient to be positioned next to the detectors and to improve resolution.
U.S. Pat. No. 5,717,212 (Fulton, et al. Feb. 10, 1998) titled “Three detector head gamma camera system with independently circumferentially positionable detector heads”, discloses a gamma camera comprising a rotating gantry, three gamma detector heads which are mounted to the rotating gantry and a linear motors selectively moves each detector head along the tracks to change its circumferential position relative to the other detector heads.
U.S. Pat. No. 5,929,446 (Plummer, et al. Jul. 27, 1999) titled “Configurable multiple detector nuclear medicine gantry” discloses a transformable gamma camera including several detectors, each of the detectors is radially movable with respect to the rotating gantry's axis of rotation.